1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the present invention relates to a semiconductor device having a portion used as a fuse.
2. Background Art
In recent years, accompanying the miniaturization, and increase in the capacity and speed of semiconductor devices, a rescuing method for securing the yield is taken in a semiconductor manufacturing process wherein spare memory cells are previously prepared in a semiconductor device, and when a defective bit is found, the defective bit is replaced by a spare memory cell. As the method for replacing the defective bit to a spare memory cell, a method wherein the portion to be used as a fuse is previously provided in a wiring layer, and a program to blow the fuse, whereby to transmit a signal to use the spare memory cell, is provided.
As a method to blow the fuse, the laser trimming system wherein laser beams are radiated onto the fuse is widely used. In this case, in general, YAG laser or YLF laser is often used to radiate laser beams.
As the material for the fuse wiring, Al, which has relatively low melting point and boiling point, is suited. The wiring used as a fuse is often formed utilizing the wiring layer used in the formation of other wirings. On the other hand, in order to form fine wirings and to reduce the wiring resistance, Cu wirings have often been used. However, since Cu has higher melting point and boiling point than Al, blowing using conventional YAG or YLF laser is difficult, and when a conventional blowing method is used, it is difficult that a Cu wiring is used as a fuse.
It is also difficult to perform Au or Al wire bonding on a Cu wiring, and Al is generally used for the uppermost wiring layer that forms the portion used as the bonding pad. A passivation film for protecting the surface of a semiconductor chip is also formed on the uppermost Al wiring, and a silicon nitride film is often used as the passivation film.
It has generally known that when an Al wiring is blown using laser beams, the Al wiring is easily cut when a silicon oxide film is formed on the Al wiring. On the other hand, since a silicon nitride film absorbs much laser beams, and has a high melting point, the blow of the Al wiring in the silicon nitride film may produce blow residues, and cannot be performed properly.
Therefore, when a silicon nitride film is used as the passivation film, it is difficult to use the Al wiring in the uppermost layer as the fuse wiring. For this reason, an Al wiring is normally formed in the silicon oxide film formed below the uppermost wiring layer to used as the fuse wiring. Generally, in order to constitute a fuse wiring, at least two layers of Al wiring layers, that is, an Al wiring layer for the bonding pad, and an Al wiring layer for the fuse formed in the silicon oxide film, are required. However, the structure wherein an insulating film is formed between the two Al wiring layers is apt to be cracked by the vibration when the wires are fixed to the bonding pad.
Although a silicon nitride film or a silicon oxide film is normally formed using a P-CVD method, the adjacent fuses may be damaged when a fuse is blown because a thin film formed using a P-CVD method is the lacking in flatness.
On the other hand, in order to use an Al wiring on the uppermost layer, and to blow the Al wiring properly, it is considered to make the Al wiring thin. The Al wiring is normally formed so as to have a thickness of 600 to 800 nm; however, if the thickness of the Al wiring is as thin as 100 to 400 nm, it is easily blown even if the Al wiring is present in the silicon nitride film. However, the reduction of the thickness of the Al wiring may lead to the deterioration of the bonding characteristics of the bonding pad, and cracks may occur in the interlayer insulating film under the Al wiring during bonding or testing, the bonding strength may lower, and the pad may be delaminated. Therefore, simply thinning the Al wiring is not preferable (e.g., refer to Patent References of Japanese Patent Laid-Open No. 2002-203902 and of Japanese Patent Laid-Open No. 2002-110799).